1. Field of the Invention
This invention relates to implants designed to be surgically inserted between the layers of the cornea to correct refractive errors. More particularly, the invention relates to corneal implants that can serve as a substitute for conventional spectacles or contact lenses.
2. Description of the Related Art
There have already been proposed artificial lenses for implantation in the eye. Such implants have hitherto been intended, not as corrective lenses, but as a substitute for the natural lens of the eye. For example, when an eye develops a cataract, the natural lens becomes fogged or opaque, thereby impairing vision. When such a cataract is treated, the lens is removed, leaving the eye aphakic. Although it is possible to correct for aphakia using spectacles, the degree of correction requires spectacles so thick as to make them both cumbersome and unattractive. For these reasons, lenses have been designed for correction of aphakia wherein the substitute lens is inserted into the eye during the operation to remove the cataract or at a second operation. Such substitute lenses are of fixed focal length and, as the natural lens has been removed, the eye is no longer capable of accommodation, that is to say, the focal length cannot change to focus at different distances. It is accepted in this field that such implanted lenses are not prescribed as an alternative to conventional spectacles for a person suffering only from myopia or presbyopia.
Another implant that has been used in the past with some success has been the artificial cornea described in U.S. Pat. No. 2,714,712, and generally resembling what is known as a kerato-prosthesis. These implants are designed as a replacement for the natural cornea itself where the cornea has become fogged or opaque, and are not intended to be a substitute for conventional spectacles or contact lenses.
It is known to resort to surgery in order to correct for defects in eyesight. The various procedures for refractive corneal surgery to correct vision problems such as myopia have not gained general acceptance in ophthalmology. These include radial keratotomy introduced in modern times (1972) by Fyodorov of the USSR, keratomileusis introduced in 1961 by Barraguer of Columbia, keratophakia which uses shaped donor corneas as lens, epikeratophakia which uses an epigraft of homologous tissues, keratotomy to correct astigmatism, and removing clear lens. Such surgery does not have a fully predictable outcome, and furthermore any non-spherical flattening of the cornea on healing results in an eyesight defect that cannot be corrected by the use of spectacles or contact lenses.
The human cornea is a transparent avascular tissue about 10-12 mm in diameter. The cornea functions as a protective membrane and as a "window" through which light rays pass en route to the retina. The average adult cornea is about 0.65 mm thick at the periphery and about 0.54 mm thick in the center (optic zone). From anterior (front) to the posterior (back), it has 5 distinct layers: the epithelium which is 5 or 6 cell layers thick; a clear acellular Bowman's layer; the stroma (which constitutes about 90% of the thickness of the cornea); the thin Descemet's membrane; and, the single layer endothelium. Sources of nutrition for the cornea are the blood vessels of the limbus, the aqueous humor and tears. The superficial cornea also gets most of its oxygen from the atmosphere.
The zone in the cornea through which incident light passes is known variously as the "optic zone" or "pupillary aperture," and both terms will be used interchangeably herein. The size of the normal pupil varies at different ages and from person to person, but normally is about 3-4 mm--smaller in infancy, tending to be larger in childhood, and again progressively smaller with advancing age. Vaughan, D., et al., General Ophthalmology, 2d ed., Appleton & Lange, Norwalk, CT, 1989, Ch. 15; Choyce, Cataract, 7 (June 1985). The size of the pupillary aperture, of course, varies inversely with the amount of incident light.
Disks of many different materials have been inserted into corneal stromal pockets, initially to control corneal edema, but more recently to correct refractive errors. Hydrogel and polysulfone lenses have been more successful than other types of lenses so far tried.
Previous corneal implants for the correction of refractive errors have enjoyed only limited success, in part because of the large diameter of the lenses used and in part because of the composition of such lenses. As will be detailed in the review of the related art below, the ophthalmologically more desirable high refractive index polymeric lenses previously used tend to prevent access of fluids, nutrients and gases such as oxygen to the tissue anterior to the implant and to the corneal tissue posterior to the implant. On the other hand, high water content, low refractive index lenses such as hydrogel lenses, while reducing or eliminating the problem of nutrient and gas transport, are generally not able to provide the necessary corrections in refractive error of the eye. Previous corneal implants have also not been able to provide multifocal refractive correction.
The large diameter of previous corneal implant lenses has also required a less-than-satisfactory surgical approach to implantation. In general, previous corneal inlays have required cutting a large pocket into the cornea and inserting in this pocket the lens which resides predominantly behind Bowman's membrane. With this type of insertion, the large implanted lens distorts the cornea, thereby producing a change in optical power. The disadvantage of such a procedure has been that the distortion is usually in the posterior side of the cornea. Such posterior distortion, however, produces only a very small change in optical power because the difference between the refractive index produced is only the small difference between the inlay/cornea and the aqueous humor.
Choyce, D.P., U.S. Pat. No. 4,607,617, issued Aug. 26, 1986, relates to an implant designed to be inserted between the layers of a cornea of an eye to correct eyesight defects, comprising a polysulfone plastic material of a high refractive index (typically 1.633), of a thickness in the range of 0.1 to 0.4 mm, and capable of being sterilized by steam autoclaving prior to insertion. As the implant is entirely embedded in the cornea, it is said not to be exposed to the atmosphere or to the aqueous humor. The polysulfone material is said to be "relatively permeable to body fluids", although it is not clear that this is so. The lens is inserted by a procedure comprising forming an incision in the outer layer of the cornea, separating layers of the cornea to form a pocket, inserting into this pocket a lens inlay, and resealing the incision. Although Choyce neither discloses nor suggests a specific diameter for the lens inlay, reference to FIG. 7b of the specification shows that this diameter is substantially greater than the optic zone of the cornea, which, as noted above, normally is about 3 mm to 4 mm in diameter. This fact, plus the fact that it is known that high refractive index plastic inlay lenses are poorly permeable to fluids, nutrient materials and necessary gases such as oxygen, limits the usefulness of this inlay lens. Further, this corneal inlay does not provide multifocality.
Grendahl, D.T., U.S. Pat. No. 4,624,699, issued Nov. 25, 1986, relates to a corneal inlay for implantation made of a plastic material such as polysulfone or PMMA. Recognizing that prior art polysulfone inlay lenses are poorly permeable to nutrients, fluids and gases, a property of concern to medicine, the inventor attempts to overcome these disadvantages by providing a corneal inlay with a plurality of holes or slots for passage of nutrients through the cornea. The inlay lens is said to have a diameter of approximately 3 mm to 7 mm, preferably 4.5 mm to 6.5 mm, more preferably slightly less than 6 mm in diameter (column 2, lines 21-26). Inlay lenses of such diameter will generally completely cover the optic zone of a normal human cornea, creating the problems of nutrient and gas supply described above. There is no disclosure or suggestion in this patent that the inlay lens could be smaller than the opening of the optic zone, nor is there reference to any property of the lens other than a single focal distance.
Lindstrom, R.L., U.S. Pat. No. 4,851,003, issued Jul. 25, 1989, discloses corneal inlay lenses applied under the cornea and about the stroma. The lens is fenestrated, and includes a plurality of fixation holes around the periphery and a coating on the anterior surface by a material that enhances the growth of corneal epithelial cells into and about the holes. The coating is composed of biological materials such as fibronectin, laminin, a glycosaminoglycan, or a type IV collagen. Although the diameter of the inlay lenses is not specifically disclosed, the dimensions of the holes (up to 1 mm), taken together with FIG. 6 which shows the epicorneal lens implanted below the epithelium, indicates that the diameter of the inlay lens is at least 5-7 mm, and thus substantially greater than the optic zone of the cornea. Such lenses also do not provide a patient with multiple focalities.
Thus, the prior art inlay lenses are less than satisfactory in important ways. Where large (e.g., 5 mm to 7 mm) hydrogel lenses are used, wherein the water content is high (about 72%) and the index of refraction low (about 1.38), problems of permeability to nutrients and gases are generally less severe, but the dioptic power is low. Where large polymeric lenses are used, wherein the water content is quite low and the refractive index high (e.g., 1.45 to 1.633), the optic power is satisfactory, but the permeability is poor. Such non-permeability to essential nutrients and gases tends to cause "starvation" in the anterior segments of the stroma, ultimately resulting in extrusion of the inserted lens. Although the permeability problem is reduced by placing holes or slots in polymeric lenses (see Grendahl above), such holes interfere with vision. Further, none of the prior art inlay lenses provide for multiple focality, which is highly desirable in many patients.
There remains, therefore, an important need for intra-corneal lenses of a refractive index sufficiently high so as to avoid the need to distort the cornea in order to obtain the desired optical power, of a size sufficiently small so as to simplify surgical insertion, of a diameter that permits essential nutrients and gases readily to reach the anterior of the cornea, and of a type that permits either unifocality or multiple focalities.
Such an intra-corneal inlay lens has been invented, and it and its uses are disclosed below.